Do power attic fans help?

[Carnak]

the best radiant barrier is a white roof

http://jacksonville.com/lifestyles/h...t_energy_costs

I guess I am lucky that every one down here knows that the metal roofing is the best for hurricanes. Easy to have a white roof. White tiles are not uncommon either

[pstu]

If you can get a house built like Carnak's that is the best solution.

Commonly new construction is done with roof decking which is foil covered on the down side. Looks just like aluminum foil. Costs little extra and no extra labor costs.

If you have a normal American house to retrofit, one option is spraying on a particular paint which is reflective and silver in color. Another option is stapling up radiant barrier foil which comes in rolls, looks just like Reynolds wrap but is much harder to tear. I have done that in my own house and it was labor intensive but worth it.

Hope this helps -- Pstu

[pomonabill220]

So....
radiant barrier stapled to rafters, foil towards roof deck (inside attic)... space on bottom near attic deck and near peak to allow for "chimney effect" air flow.?
Thx

[Carnak]

If you can get a house built like Carnak's that is the best solution.

or if you have a house like Dad's

Did Ike leave most roofs intact?

After Ivan a lot of people here re-did their rooves a lot smarter than they had before

[pstu]

Ike damaged a lot of roofs but I don't remember hearing about many torn off. It was not the strongest hurricane. Among the people who don't believe in keeping up the house, you can still see many of those distinctive blue "FEMA" tarpaulins keeping out the weather.

Regards -- Pstu

[pstu]

So....
radiant barrier stapled to rafters, foil towards roof deck (inside attic)... space on bottom near attic deck and near peak to allow for "chimney effect" air flow.?
Thx

With the ventilated attic paradigm, you pretty much have it right.

Most RB foil is 2-sided, but if yours is 1-sided better to have it facing down, with at least a 1-inch air space in front of it. Minor issue, not much of a pitfall.


About the other paradigm... some building pros around here claim that a foamed roof deck is likely to lead to rotting decking. Allegedly this comes from some local experience and research done by Texas A&M. The local experience is just anecdotal and hard to trace, but the A&M research ought to be fully public. I'll see if I can locate it.

Regards -- Pstu

[pomonabill220]

Great! Thanks for the clarification!

[MCHUTZ]

Do power attic fans do a better job of venting the heat out? I have heard some people say they are a waist of money because the motors won't last long running in the heated condition they run in. I have 3 in my attic and all 3 motors are done. I want to replace them with either a higher quality moter or take them out completly and use something else. I live in Tennessee so our arttics here get pretty hot.

Attics here in the southern states (TX, FL, GA, TN, AZ, LA, etc.) are more than hot. Most of us that have spent anytime working in an attic know they are a miserable environment to work in. However, a properly ventilated attic makes life not only better for us HVAC contractors, but also provides a much better living environment for the homes occupants. The keys are to have a trifecta of:
1) Proper insullation
2) Some sort of radiant barrier to minimize solar absorption
3) Good ventilation

We have designed a Solar Powered Attic Fan (Attic Cool 1600) that provides up to 1600 CFM of attic air displacement. When properly sized (1 Unit to every 2300 square feet of attic space) products such as these can achieve 10 air circulations each hour (As required by the home ventilating institute - HVI) to achieve energy efficiency. The KEY to PAV's (Powered Attic Ventilators) working correctly include:
1) Use of a high, quality product. Our Solar Powered AC1600 uses a variable speed, brushless motor that is encased in stainless steel. As such, the product has a 25 year, transferrable warranty. No more motor burn outs every 2 to 3 years. We use a proprietary poly solar panel. Although monocrystalline are generally more efficient, that is not the case for stand alone applications. Polycrystalline panels are remarkable at generating power in low light, cloudy or partially shaded environments. We could not generate the same results with monocrystalline panels. There are numerous other features to the Attic Cool 1600 that make it a great product for solar powered attic ventilation, but there is more to proper attic ventilation than this.
2) You MUST ensure that you have adequate passive or static ventilation in place prior to installing any PAV. The HVI recommends a minimum of one square foot of soffit ventilation for every 300 CFM of displacement by the PAV. We have found this to be a little low, and recommend 2 sqaure feet for every 300 CFM. The worst thing that can happen with PAV installation is to create a negative pressure situation that could draw cool air from your air conditioned living space.
3) The Solar PAV needs to be properly positioned at or near the top of the roof, as far from the majority of the air intakes as possible. It is also important to place the unit away from roof mounted air intakes, such as air hawks, whirly birds, etc. as it may result in drawing the majority of clean air in through the roof mounted vents as opposed to the soffit vents - which are the optimal conduit for new attic air.

The bottomline, as with any HVAC associated product, proper pre-installation inspections and proper installation with skilled technicians generally leads to better results. When used properly, Solar Powered Attic Vents (with intgrated thermostats) can provide years of benefit to the home owner, including:
1) Preservation of attic placed mechanical equipment
2) Preservation of duct work
3) Extending the useful life of the roof and roof decking
4) Better indoor temperature control

All of which add up to both short and long term savings!

[Carnak]

CHutzie

you vent an attic to stop icycles

[pstu]

Attics here in the southern states (TX, FL, GA, TN, AZ, LA, etc.) are more than hot. Most of us that have spent anytime working in an attic know they are a miserable environment to work in. However, a properly ventilated attic makes life not only better for us HVAC contractors, but also provides a much better living environment for the homes occupants. The keys are to have a trifecta of:
1) Proper insullation
2) Some sort of radiant barrier to minimize solar absorption
3) Good ventilation

We have designed a Solar Powered Attic Fan (Attic Cool 1600) that provides up to 1600 CFM of attic air displacement. When properly sized (1 Unit to every 2300 square feet of attic space) products such as these can achieve 10 air circulations each hour (As required by the home ventilating institute - HVI) to achieve energy efficiency. The KEY to PAV's (Powered Attic Ventilators) working correctly include:
1) Use of a high, quality product. Our Solar Powered AC1600 uses a variable speed, brushless motor that is encased in stainless steel. As such, the product has a 25 year, transferrable warranty. No more motor burn outs every 2 to 3 years. We use a proprietary poly solar panel. Although monocrystalline are generally more efficient, that is not the case for stand alone applications. Polycrystalline panels are remarkable at generating power in low light, cloudy or partially shaded environments. We could not generate the same results with monocrystalline panels. There are numerous other features to the Attic Cool 1600 that make it a great product for solar powered attic ventilation, but there is more to proper attic ventilation than this.
2) You MUST ensure that you have adequate passive or static ventilation in place prior to installing any PAV. The HVI recommends a minimum of one square foot of soffit ventilation for every 300 CFM of displacement by the PAV. We have found this to be a little low, and recommend 2 sqaure feet for every 300 CFM. The worst thing that can happen with PAV installation is to create a negative pressure situation that could draw cool air from your air conditioned living space.
3) The Solar PAV needs to be properly positioned at or near the top of the roof, as far from the majority of the air intakes as possible. It is also important to place the unit away from roof mounted air intakes, such as air hawks, whirly birds, etc. as it may result in drawing the majority of clean air in through the roof mounted vents as opposed to the soffit vents - which are the optimal conduit for new attic air.

The bottomline, as with any HVAC associated product, proper pre-installation inspections and proper installation with skilled technicians generally leads to better results. When used properly, Solar Powered Attic Vents (with intgrated thermostats) can provide years of benefit to the home owner, including:
1) Preservation of attic placed mechanical equipment
2) Preservation of duct work
3) Extending the useful life of the roof and roof decking
4) Better indoor temperature control

All of which add up to both short and long term savings!

I would like to know where the HVI number of 10 ACH was derived. Were there any papers discussing the merits of higher or lower numbers? If you could provide any links that would be helpful.

Even with 1:300 or 1:150 soffit ventilation, can you really believe makeup air will all come from the soffits and not from ceiling/attic leaks? Remember air is stupid and will go anywhere there is a pressure differential. Have there been any measurements as to the degree of negative pressure created within the attic?

Carnak is correct the original emphasis on attic ventilation was to prevent ice buildup in colder climates. Seems like most of building science started thinking about the industrial North America, and only later extended their thinking to hot-humid climates. I am aware of the arguments in favor of a sealed and conditioned attic, however there is a counter- philosophy which will not go away. I believe their arguments are in general weaker, however I've not been able so far to hear a good articulation about why attics should be ventilated. Despite the philosophical ideal, I think the market share of ventilated attics is over 99%, that represents a lot of houses which need to be tended in a realistic (vs. idealistic) way.

Thank you -- Pstu

[Kevin O'Neill]

Joe Lsteburek of Building Science Corp has written a book about building homes in hot, humid climates. I have a copy somewhere. As I recall, he is generally against powered attic fans. I will see if I can find the book.

[Shophound]

When energy prices rise again - and they will - folks will get more serious about more intense options to reduce heat gain to an attic. Solar powered PAVs seem viable only if an airtight seal can be assured between attic and house. What's the use of keeping an attic cooler if it comes at the expense of increasing infiltration into the house? It's a dog chasing his tail in circles, which we already have WAY too much of in the HVAC and construction/retrofit industries.

[pstu]

When energy prices rise again - and they will - folks will get more serious about more intense options to reduce heat gain to an attic. Solar powered PAVs seem viable only if an airtight seal can be assured between attic and house. What's the use of keeping an attic cooler if it comes at the expense of increasing infiltration into the house? It's a dog chasing his tail in circles, which we already have WAY too much of in the HVAC and construction/retrofit industries.

Again: what is wrong with using attic fans in pairs, one supplying "cooler" air to the attic from a favored location, the other in the expected place to suck hotter air out from the top of the attic? Why wouldn't this eliminate the objection of negative pressure in the attic?

Hope this helps -- Pstu